This invention relates to the polymerization of 2-pyrrolidone and more particularly to a practical method of decreasing the time required for the formation of the polymer while still preserving the high molecular weight and white color of the product.
The formation of polymers of 2-pyrrolidone involving the use of alkaline catalysts via an anionic mechanism is disclosed in U.S. Pat. No. 2,638,463. Subsequent patents, for example U.S. Pat. No. 2,809,958, further disclose the need for an activator or cocatalyst to increase the yield of polymer formed.
Suitable alkaline catalysts are the oxides, hydroxides, alcoholates, hydrides, amides, etc. of the alkali metals, as well as the alkali metals themselves, which form the alkali metal salt of 2-pyrrolidone ##STR2## which is the actual catalyst.* FNT *As used herein "alkali metal" means all the alkali metals except lithium.
The simplest, and most free from side reactions, are the alkali metal hydroxides.
The polymer formed is believed to be a linear polyamide which has been called polypyrrolidone or nylon-4, having the structure: ##STR3## As is well known in the art, this polymer may be melt extruded to form useful articles such as rods, films, molded objects and filaments including textile fibers where its unique moisture regain properties are especially advantageous.
The value of n, or in other words the molecular weight of the polymer may vary from under 100,000 to over 1 million, depending upon the type of activator or initiator employed but oligomers are never formed. The vast majority of activators which have been disclosed in the prior art (for example those disclosed in U.S. Pat. Nos. 2,809,958; 3,060,153 and 3,069,392) result in a polymer of medium molecular weight having insufficient thermal stability to withstand the high temperatures required for melt extrusion processes. It has been found that in order to be satisfactory for this purpose the polymer should have a molecular weight high enough to exhibit an inherent viscosity of at least 3.0 when measured as a 0.5% solution in hexafluoroisopropanol at 25.degree. C. and should also have a narrow molecular weight distribution indicated by a polydispersity value of 5 or under. A satisfactory test for melt extrudability is disclosed in U.S. Pat. No. 3,721,652 Column 12, Examples 5(a) and 5(b).
Of the numerous activators which have been disclosed for initiating the polymerization of 2-pyrrolidone, two especially result in markedly higher molecular weights and low polydispersity values. One of these is carbon dioxide, disclosed in U.S. Pat. No. 3,721,652 and the other is sulfur dioxide, disclosed in U.S. Pat. No. 3,174,951. While the molecular characteristics are similar, polymers made using SO.sub.2 as the chain initiator usually have a yellow color which greatly limits their usefulness while those made via CO.sub.2 are white. A method of forming white polymers when SO.sub.2 is used as the activator is disclosed in our co-pending application, now U.S. Pat. No. 4,105,645 issued Aug. 8, 1978 which is hereby incorporated by reference.
Typically, the polymerization of 2-pyrrolidone to form a product of sufficiently high molecular weight to be useful in melt extrusion processes requires long polymerization times. For example, U.S. Pat. No. 3,721,652 recites a polymerization time of 5 days at 50.degree. C. to give a conversion of 55% (Example 6) and Gunter Schirawski (Die Makromolekulare Chemie 161 page 64 Table 6) reports a conversion of 52.2% at 50.degree. C. in 3 days when using an optimum CO.sub.2 concentration. By optimizing all of the variables it is possible to shorten the polymerization time even further, but at least about 30 hours is required in order to obtain conversions of 50-55% when using either CO.sub.2 or SO.sub.2 as the activator and either sodium or potassium pyrrolidonate as the primary catalyst.
It is well known that by using a quaternary ammonium hydroxide instead of an alkali metal hydroxide, the time required for substantial yields of polymer can be greatly reduced. The first disclosure of the use of quaternary ammonium hydroxides as primary catalysts was by Ney in U.S. Pat. No. 2,973,343. Shorter polyermization times were reported together with an increase in molecular weight, even when using N-acetyl pyrrolidone as the activator. In Example 9 of U.S. Pat. No. 3,721,652, the use of a quarternary ammonium hydroxide catalyst together with CO.sub.2 is disclosed. Polymerization times as low as 5 hours for a conversion of 50% were reported together with higher molecular weights.
There is, however, a serious problem in the use of quaternary ammonium hydroxides as catalysts. They are very unstable when heated, especially under the reduced pressure which must be used to prepare the pyrrolidone salt, breaking down into tertiary amine and alcohol or olefin. Because of this it is difficult to prepare even modest size batches for polymerization. Note that in the aforementioned Example 9 of U.S. Pat. 3,721,652 only 25 ml of monomer was employed whereas in Example 4 of the same patent, 800 cc of monomer was used.
Sekiguichi et al, as disclosed in U.S. Pat. No. 3,835,100, attempted to get around this problem by preparing the pure, anhydrous quaternary ammonium salts of 2-pyrrolidone. Since these compounds are in fact the actual primary catalysts they may be added to the pyrrolidone without heating and no loss by decomposition will occur.
The difficulty with this solution to the problem is that these quaternary ammonium salts of pyrrolidone are very expensive to prepare and they must be kept "in dry state, under vacuum, and in the cold" (column 2, line 44 of U.S. Pat. No. 3,835,100) since they are very hygroscopic and unstable to heat.
After the filing date of our original application U.S. Pat. No. 4,098,774 issued in which the use of quaternary ammonium halides to form the quaternary ammonium salt of 2-pyrrolidone in situ is disclosed. Polymerization rates similar to those obtained with the quaternary ammonium salt of 2-pyrrolidone are disclosed.
In this way the problem associated with forming the quaternary ammonium salt via the thermally unstable quaternary ammonium hydroxide are also avoided since the dry quaternary ammonium halide is added to the anhydrous polymerizate directly from a dry box whereby the quaternary ammonium salt of 2-pyrrolidone is believed to be formed in situ. without heating.
We also tried adding anhydrous quaternary ammonium halides to the polymerizate using both CO.sub.2 and SO.sub.2 as activators and found them to be as effective as the quaternary ammonium salt of 2-pyrrolidone when carefully prepared but no more so. But during the course of our investigation we tried quaternary ammonium compounds other than the halides and found, much to our surprise, that certain rather specific ones induce polymerization rates which are very much faster than those of the halides. For example the highest conversion disclosed in U.S. Pat. No. 4,098,774 is 69.1% after 22 hours at 50.degree. C. when equimolar amounts of tetramethyl ammonium chloride and carbonated potassium pyrrolidonate were employed. Without the addition of the tetramethyl ammonium chloride the conversion was 45.2%. This is a polymerization rate of about 2% per hour; the addition of the tetramethyl ammonium chloride raised this rate to about 3% per hour.
Since the polymerization rate typically slows down as higher conversions are reached, somewhat higher rates prevail earlier in the polymerization. Thus after 8 hours using the same concentrations, the above cited patent indicates a conversion of 40% when the tetramethyl ammonium chloride is added versus 16.3% when only the corresponding amount of carbonated potassium pyrrolidonate is present. The addition of the quaternary ammonium halide increased the polymerization rate from about 2% to about 5% per hour.
The fastest rate disclosed in the patent is 6.7% per hour (Example 6a) when 10 mol percent of the carbonated potassium salt was used. This gave a conversion of 53.9% in 8 hours although after 22 hours the conversion was almost 10% lower than that obtained when 5 mol percent of the carbonated potassium salt was used (Example 5b).
In marked contrast to these rates we have found that it is possible to achieve a polymerization rate of more than 50% per hour when using some of the accelerators of this invention while still preserving molecular weight characteristics suitable for melt extrusion processes. The preferred quaternary ammonium compounds of our invention are very specific and may be represented by the following formula: ##STR4## where R.sub.1, R.sub.2, and R.sub.3 are n-propyl, n-butyl or n-amyl groups and R.sub.4 is a methyl, ethyl, n-propyl, n-butyl or n-amyl group, X is either a sulfate or bisulfate (i.e. hydrogen sulfate) and n is either 1 or 2 depending on whether X is a bisulfate or sulfate.
Although the rates are slower, quaternary ammonium sulfates containing alkyl groups lower then propyl, i.e. ethyl or methyl, are operable. For example tetramethyl ammonium sulfate will give a conversion of 41% in 4 hours at 50.degree. C. using SO.sub.2 as the activator, equivalent to just over 10% per hour. The fastest rate for tetramethyl ammonium chloride disclosed in U.S. Pat. No. 4,098,774 is 6.7% per hour (Example 6a). By way of comparison, tetrabutyl ammonium sulfate can give a rate of conversion equal to 18.9% per hour together with a molecular weight of over 1 million. This effect of the tetrabutyl group would not be expected from the results obtained with the halides. For example in Table VI of the aforementioned patent tetrabutyl ammonium iodide is listed as giving 23.9% conversion in 8 hours compared with tetremethyl ammonium iodide which gave 33% conversion in the same period of time. It would not be predicted that the effect would be the reverse when using the sulfates and at the same time much greater in magnitude.
It should be noted that while very high polymerization rates may be obtained by using a "co-activator" system such as disclosed by Jarovitzky in U.S. Pat. No. 3,681,295 and also in Examples 13 and 14 of U.S. Pat. No. 4,098,774, these methods produce polymers which are not suitable for melt extrusion processes. They basically are co-activated with N-acetyl pyrrolidone or other N-acyl compound, which results in the polymer formed consisting of two different molecular species. A gel permeation chromatogram of such polymers is bilobal, exhibiting a low molecular weight peak typical of N-acyl activated polymer and a higher molecular weight peak which results from the CO.sub.2 initiated polymer (U.S. Pat. No. 3,721,652 Col. 6, lines 10-16 and Chemtech, Jan. 1972 page 17). Average molecular weights are therefore misleading. GPC curves show that polymer formed by the polymerization accelerators disclosed in this invention consists of one species only and is very narrow in molecular weight distribution, having polydispersity values (A.sub.w /A.sub.n) of 2 to 3.
For good performance in melt extrusion processes it is this low polydispersity value which is of greatest importance. Higher molecular weights simply result in higher melt viscosity in the extruder which, although not a problem, are not especially beneficial. Polymers of 2-pyrrolidone having high (i.e. over 1 million), medium or low (i.e. about 150,000) molecular weights may be made by methods disclosed herein, all having low polydispersity values.
The utility of our invention lies in the greatly increased rates of polymerization not heretofore achieved. This is of great importance commercially since it permits increased output from a plant without enlarging the equipment thereby lowering the cost of production. The slower rates resulting from the use of the quaternary ammonium halides does not lower production costs enough to compensate for the cost of adding them.
In practicing this invention CO.sub.2, SO.sub.2 or MoO.sub.3 may be used as activators to form polymers of 2-pyrrolidone having a low polydispersity. If this is unimportant for the intended application other activators may be used and even faster rates of polymerization attained.
It is an object of this invention to provide polymerization accelerators having greatly increased activity thus increasing the polymerization rate substantially beyond that achieved with ordinary accelerators while preserving a low dispersity ratio in the resulting polymer.
It is another object of the invention to provide a novel method of drying the quaternary ammonium salt prior to its addition to the polymerization mixture.
It is a further object of this invention to provide a method of maintaining pure white polymers when polymerizing 2-pyrrolidone with SO.sub.2 at higher temperatures.
It is a still further object of the invention to provide a method of recycling both unchanged monomer and the polymerization accelerator.
Other objects will be apparent from the more detailed disclosure.